Papaya (Carica papaya L.) is an important fruit crop grown widely in tropical and subtropical lowland regions (Manshardt, “Papaya in Biotechnology of Perennial Fruit Crops,” ed. Hammerschlag, 21:489-511, CAB Int., Wallingford, UK (1992)). Worldwide, Brazil, India, and Mexico are the largest producers of papaya. Hawaii, the largest producer of papaya in the United States, exporting about 66% of the total fresh production, primarily to the US mainland and Japan (Martin, “Papaya Production Statistics,” Proc. Annu. Hawaii Papaya Ind. Assoc. Conf., 39th, Kihei, pp. 31-36, Sept. 23-24 (1994)). The FAO estimated that about 5.7 million metric tons of fruit were harvested in 1995, almost double the 1980 harvest (Galinsky, “World Market for Papaya,” Reg. Agribus. Proj. Mark. Inf. Bull. Feb. No. 12, 5 pp. (1996)).
Papaya ringspot virus (“PRSV”) is a member of the potyvirus group of plant viruses, which are pathogenic to several crop plants, and which exhibit cross-infectivity between members of different plant families. Generally, a potyvirus is a single-stranded (+) RNA plant virus. The viral genome is approximately 10,000 bases in length. The expression strategy of potyviruses includes translation of a complete polyprotein from the positive sense viral genomic RNA. PRSV is by far the most widespread and damaging virus that infects papaya, occurring worldwide wherever papaya is grown (Purcifull, “Papaya Ringspot Virus,” CMI/AAB Descr. Plant Viruses, No. 292 (No. 84 Revis., July 1984) 8 pp. (1984)). PRSV infections have resulted in the devastation of the papaya industry in Brazil, Taiwan, and Hawaii in recent years (Gonsalves, D., “Control of Papaya Ringspot Virus in Papaya: A Case Study,” Annu. Rev. Phytopathol. 36:415-37 (1998)). Various attempts have been made to control or prevent infection of crops by PRSV, but these have been largely unsuccessful.
The concept of parasite-derived resistance (“PDR”), conceived in the middle 1980s, offered a new approach for controlling PRSV (Sanford et al., “The Concept of Parasite-Derived Resistance—Deriving Resistance Genes from the Parasite's Own Genome,” J. Theor. Biol. 113:395-405 (1985)). Parasite-derived resistance is a phenomenon whereby transgenic plants containing genes or sequences of a parasite are protected against detrimental effects of the same or related pathogens. (Powell-Abel et al., “Delay of Disease Development in Transgenic Plants that Express the Tobacco Mosaic Virus Coat Protein Gene,” Science, 232:738-43 (1986); (Lomonossoff, G. P., “Pathogen-Derived Resistance to Plant Viruses,” Ann. Rev. Phytopathol. 33:323-43 (1995)).
The vast majority of reports regarding PDR have utilized the coat protein (“CP”) genes of the viruses that are targeted for control (Powell-Abel et al., “Delay of Disease Development in Transgenic Plants that Express the Tobacco Mosaic Virus Coat Protein Gene,” Science, 232:738-43 (1986)); however, a growing number of reports have shown that viral replicase (Golemboski et al., “Plants Transformed with a Tobacco Mosaic Virus Nonstructural Gene Sequence are Resistant to the Virus,” Proc. Natl. Acad. Sci. USA 87:6311-15 (1990)), movement protein (Beck et al., “Disruption of Virus Movement Confers Broad-Spectrum Resistance Against Systemic Infection by Plant Viruses with a Triple Gene Block,” Proc. Natl. Acad. Sci. USA 91:10310-14 (1994)), nuclear inclusion a-proteases (“NIa proteases”) of potyviruses (Maiti et al., “Plants that Express a Potyvirus Proteinase Gene are Resistant to Virus Infection,” Proc. Natl. Acad. Sci. USA 90:6110-14 (1993)), and other viral genes are also effective in conferring resistance. Furthermore, viral genes can be effective in the translatable and non-translatable sense forms, and, less frequently, antisense forms (Baulcombe, D. C., “Mechanisms of Pathogen-Derived Resistance to Viruses in Transgenic Plants,” Plant Cell 8:1833-44 (1996); Dougherty et al., “Transgenes and Gene Suppression: Telling us Something New?” Current Opinion in Cell Biology 7:399-05 (1995); Lomonossoff, G. P., “Pathogen-Derived Resistance to Plant Viruses,” Ann. Rev. Phytopathol. 33:323-43 (1995)).
Although the testing of transgenic plants have been largely confined to laboratory and greenhouse experiments, a growing number of reports showed that resistance is effective under field conditions (Grumet, R., “Development of Virus Resistant Plants via Genetic Engineering,” Plant Breeding Reviews 12:47-49 (1994)). Two virus resistant crops have been deregulated by APHIS/USDA and, thus, are approved for unrestricted release into the environment in the U.S.A. Squash that are resistant to watermelon mosaic virus 2 and zucchini yellow mosaic potyviruses have been commercialized (Fuchs et al., “Resistance of Transgenic Hybrid Squash ZW-20 Expressing the Coat Protein Genes of Zucchini Yellow Mosaic Virus and Watermelon Mosaic Virus 2 to Mixed Infections by Both Potyviruses,” Bio/Technology 13:1466-73 (1995); Tricoli, et al., “Field Evaluation of Transgenic Squash Containing Single or Multiple Virus Coat Protein Gene Constructs for Resistance to Cucumber Mosaic Virus, Watermelon Mosaic Virus 2, and Zucchini Yellow Mosaic Virus,” Bio/Technology 13:1458-65 (1995)). A transgenic Hawaiian papaya that is resistant to PRSV has also been developed (Fitch et al., “Virus Resistant Papaya Derived from Tissues Bombarded with the Coat Protein Gene of Papaya Ringspot Virus,” Bio/Technology 10:1466-72 (1992); Tennant et al., “Differential Protection Against Papaya Ringspot Virus Isolates in Coat Protein Gene Transgenic Papaya and Classically Cross-Protected Papaya,” Phytopathology 84:1359-66 (1994)). This resistant transgenic papaya was recently deregulated by the Animal and Plant Health Information Service of the United states Department of Agriculture (“USDA/APHIS”). Deregulation of the transgenic papaya is timely, because Hawaii's papaya industry is being devastated by PRSV. Remarkable progress has been made in developing virus resistant transgenic plants despite a poor understanding of the mechanisms involved in the various forms of pathogen-derived resistance (Lomonossoff, G. P., “Pathogen-Derived Resistance to Plant Viruses,” Ann. Rev. Phytopathol. 33:323-43 (1995)).
Unfortunately, the papaya grower faces a second natural challenge that threatens to limit the growth of the industry: the fragility of the papaya fruit. The characteristic fragility of ripe papaya fruit limits the large-scale exportation of mature papaya to countries in temperate regions. To minimize this problem, the current practice is to collect fruits for exportation in very precocious phases of maturation with the consequence of adulteration of the organoleptic characteristics of this fruit. This early harvest of fruit, designed to avoid damage in subsequent handling, can result in a failure to develop optimum fruit flavor and color. Another tactic is employed to slow the ripening process in-transit by shipping and storing papaya at cold temperatures. This practice ultimately results in significant fruit damage also, as papaya fruit is susceptible to chilling injury, with critical temperatures ranging between 10-15° C. In papaya, the symptoms of chilling injury are more evident upon returning the fruit to higher ripening temperatures, which results in excessive softening and the associated enhancement of pathogen susceptibility (Chan et al., “Electrolyte Leakage and Ethylene Production Induced by Chilling Injury of Papayas,” Hort. Science 20:1070-1072 (1985); Lyons et al., “Chilling Injury,” in Weichmann, ed., Postharvest Physiology of Vegetables, New York: Marcell Dekker Inc., pp. 305-326, (1987)).
In an effort to solve the problems associated with long-distance shipping of fruit generally, researchers have concentrated on unraveling the role of enzymes involved in the ripening process. Three enzymes that have surfaced as vital for fruit ripening are pectinmethylesterase (“PME”), β-glucuronidase (“β-Gal”), and the polygalacturonase (“PG”) family.
PME is a pectolytic enzyme which has been implicated in fruit ripening (Bacic et al., “Structure and Function of Plant Cell Walls,” in The Biochemistry of Plant: A Comprehensive Treatise, ed. J. Preiss, 14:297-371, New York: Academic (1988)). This cell wall metabolizing enzyme is responsible for the demethylation of galacturonic acid residues in high molecular weight pectin, each methyl group being converted to a proton and methanol (Hall et al., “Molecular Characterization of cDNA Clones Representing Pectin Esterase Isozymes from Tomato,” Plant Mol. Biol. 25(2):313-318 (1994)). PME activity has been reported to increase during the development of banana (Brady, “The Pectinesterase of Pulp Banana Fruit,” Aust. J. Plant Physiol. 3:163-172 (1976)), apple (Knee, “Metabolism of Polygalacturonase in Apple Fruit Cortical Tissue During Ripening,” Phytochemistry 17:1262-1264 (1979)), avocado (Awad et al., “Postharvest Variation in Cellulase, Polygalacturonase and Pectin Methylesterase in Avocado (Persea americana) Fruit in Relation to Respiration and Ethylene Production,” Plant Physiol. 64:306-308 (1979)), and papaya (Paul et al., “Postharvest Variation in Cell Wall Degrading Enzymes of Papaya (Carica papaya) During Ripening,” Plant Physiol. 72:382-385 (1983)). The exact role of PME in fruit development and ripening is yet to be determined. However, it has been hypothesized that de-esterification of pectin by PME and further depolymerization by PG are involved in fruit softening. This hypothesis is based on the observation that demethylation of pectin by PME causes a several-fold increase in cell wall solubilization by PG (Pressey et al., “Solubilization of Cell Wall by Tomato Polygalacturonase Effects of Pectinesterase,” J. Food Biochem. 6:57-74 (1982)).
A wide range of enzymes is known to catalyze aspects of pectin modification and disassembly. Among those best characterized are exo- and endo-polygalacturonases (“PGs”), which are implicated in the disassembly of pectin that accompanies many stages of plant development, in particular those requiring cell separation. Although being clear that PG participates in a wide range of developmental processes, the majority of research has been focused on its role in fruit ripening.
PG-dependent disassembly has been most extensively studied in ripening tomatoes. Following the experiences of suppression of PG gene expression in wild type tomato and on the ectopic expression of PG in the ripening impaired pleiotropic mutant ripening inhibitor (“rin”), it has been considered that PG-mediated pectin depolymerization is not necessary for normal ripening and softening (Sheehy et al., “Reduction of Polygalacturonase Activity in Tomato Fruit by Antisense RNA,” Proc. Natl. Acad. Sci. USA 85:8805-8809 (1988); Smith et al., “Antisense RNA Inhibition of Polygalacturonase Gene Expression in Transgenic Tomatoes,” Nature 334:724-726 (1988); Giovannoni et al., “Expression of a Chimeric Polygalacturonase Gene in Transgenic Rin (Ripening Inhibitor) Tomato Fruit Results in Polyuronide Degradation But Not Fruit Softening,” Plant Cell 1:53-63 (1989)). Research performed with transgenic sense and antisense tomatoes suggests that PG-mediated pectin disassembly does not contribute to early fruit ripening but contributes to tissue deterioration in the late stages of fruit ripening (Hadfield et al., “Polygalacturonase Gene Expression in Ripe Melon Fruit Supports a Role for Polygalacturonase in Ripening-Associated Pectin Disassembly,” Plant Physiol. 117: 363-373 (1998)). Analysis of cell walls from transgenic fruits with altered levels of PG activity led to the conclusion that pectin depolymerization and pectin solubilization are due to distinct enzymatic determinants (Hadfield et al., “Polygalacturonase: Many Genes in Search of a Function,” Plant Physiol. 117:337-343 (1998)). According to the same authors, pectin solubilization is primarily due to the action of PG. The fact that pectins in PG-complemented rin fruit are both solubilized and depolymerized accounts for the conclusion that PG activity is necessary and sufficient for pectin depolymerization, but it may be one of multiple, redundant pectin-solubilizing activities (Hadfield et al., “Polygalacturonase: Many Genes in Search of a Function,” Plant Physiol. 117:337-343 (1998)).
In papaya, the gradual firmness loss of fruit is associated with a discernible, although very limited, increased in PG activity (Ali et al., “The Biochemical Basis of Accelerated Softening in Papaya Following Storage at Low Temperature,” Acta Horticulture 343 (1993)). In contrast, other fruits such as strawberry (Fragaria ananassa) (Huber, “Strawberry Fruit Softening: The Potential Roles of Polyuronides and Hemicelluloses,” J. Food Sci. 49:1310-1315 (1984)), melon (Cucumis melo) (McCollum et al., “Modification of Polyuronides and Hemicelluloses During Muslanelon Fruit Softening,” Physiol. P1. 76:303-308 (1989)), and persimmon (Diospyrus kaki) (Cutillas-Iturralde et al., “Metabolism of Cell Wall Polysaccharides from Persimmon Fruit: Solubilization During Fruit Ripening Occurs in Apparent Absence of Polygalacturonase Activity,” Physiol. Plant. 89:369-375 (1993)) have been reported as lacking endo-PG activity. Recently, it was demonstrated that PG mRNA accumulation can occur at late stages of ripening at levels much lower than those observed in ripening tomato, only detectable by using very accurate methods (Wu et al., “Endopolygalacturonase in Apples (Malus domestica) and its Expression During Fruit Ripening,” Plant Physiol. 102:219-225 (1993)). It has also been reported that of three genes encoding melon PGs, one of those (MPG1) encodes an endo-PG with the potential to depolymerize melon fruit cell wall pectin (Hadfield et al., “Polygalacturonase Gene Expression in Ripe Melon Fruit Supports a Role for Polygalacturonase in Ripening-Associated Pectin Disassembly,” Plant Physiol. 117:363-373 (1998)). It is therefore possible that in some fruits the disassembly of pectins in late stages of ripening is PG dependent, even in fruits with very low levels of PG activity (Hadfield et al., “Polygalacturonase: Many Genes in Search of a Function,” Plant Physiol. 117:337-343 (1998)).
Another enzyme that has been implicated in fruit ripening is β-Gal, an enzyme involved in cell wall softening and known to exist in three isoforms (β-Gal I, β-Gal II, and β-Gal III). In “β-Galactosidases in Ripening Tomatoes,” Plant Physiol. 71:132-135 (1983), Pressey et al., reported on the increase of activity of one of the three β-galactosidases isozymes during tomato ripening, suggesting that these isozymes may play a role on degradation of cell wall galactan, which may account for the involvement of β-Gal in fruit softening. The involvement of β-Gal in tomato fruit ripening has been confirmed (Watkins et al., “Activities of Polygalacturonase α-D Mannosidase and α-D and β-D Galactosidases in Ripening Tomato,” Hortscience 23: 192-94 (1988)). More recently, the increase of β-Gal during ripening of kiwi fruit (Wegrzyn et al., “Pectinesterase, Polygalacturonase and β-Galactosidase During Softening of Ethylene-Treated Kiwi Fruit,” HortScience 27:900-902 (1992)), mango and papaya (Lazan et al., “Cell Wall Hydrolases and Their Potential in the Manipulation of Ripening of Tropical Fruits,” Asean Food J. 8:47-53 (1993)), avocado (De Veau et al., “Degradation and Solubilization of Pectin by β-Galactosidases Purified from Avocado Mesocarp,” Physio. Plant 87:279-285 (1993)), and coffee (Golden et al., “β-Galactosidase from Coffea arabica and its Role in Fruit Ripening,” Phytochemistry 34:355-360 (1993)) have been reported. In apples, the loss of fruit firmness during ripening has been associated with increased activity of β-galactosidase and a decrease in the Gal content of the cell wall (Bartley, “β-Galactosidase Activity in Ripening Apples,” Phytochemistry 13:2107-2111 (1974); Wallner, “Apple Fruit β-Galactosidase and Softening in Storage,” J. Am. Soc. Hort. Sci. 103:364 (1978)). Furthermore, Kang et al., “N-Terminal Amino Acid Sequence of Persimmon Fruit β-galactosidase,” Plant Physiol. 105:975-979 (1994) purified two isozymes (one 34 kD and the other 44 kD) from persimmon fruit. A characteristic feature during the ripening of papaya fruit is softening. β-galactosidase might contribute significantly to pectin and hemicellulose modification and, hence, to softening of the fruit (Lazan et al., “β-galactosidase, Polygalacturonase and Pectinesterase in Differential Softening and Cell Wall Modification During Papaya Fruit Ripening,” Physiol. Plant 95:106-112 (1995)).
According to Ali et al., “The Biochemical Basis of Accelerated Softening in Papaya Following Storage at Low Temperature,” Acta Horticulture 343 (1993), PME, PG, and the β-Gal isoforms may collectively play a significant role in the development of the chilling injury symptom of increased-susceptibility-to-disease commonly observed in papaya upon returning chill-stored fruits to warmer environments. Attempts to deliver mature, full-flavored, and unadulterated papaya fruits to the consumer by long-distance transport have concentrated thus far on largely unsuccessful measures such as early harvest and low temperature storage. Given the complexity of the ripening process in papaya, it not surprising that delivering mature, full-flavored, and unadulterated papaya fruits using such measures as early harvest and low temperature storage have been largely unsuccessful.
The papaya industry is doubly vulnerable: first, to the potential for wholesale destruction from PRSV infection and, second, to unremediable damage to the fruit in long-distance transport to consumers. What is needed is a solution which utilizes and adapts the natural maturation process of the papaya such that the fruit can tolerate the stresses of long-distance exportation, carried out in combination with a method to confer PRSV resistance to papaya plants.
The present invention is directed to overcoming these and other deficiencies in the art.